The present invention relates to a vehicle steering system capable of controlling the steered angle or the steerable wheels independently of steering operation of the driver.
While the vehicle is running, there occurs frequent changes in external factors such as road surface conditions, wind direction and so, which may change the vehicle running and steering conditions abruptly. The driver continues to manipulate the steering wheel in an appropriate manner so as to maintain a stable running condition. For instance, when the vehicle is running at high speeds, lateral wind force acting on one side of the vehicle may obstruct smooth straight ahead running of the vehicle. The driver should endeavor to keep the vehicle running straight ahead against the lateral wind (external factor), which is tiresome.
In view of this, various attempts have been made in recent years to develop a steering system, which is capable of assisting the driver""s vehicle maneuvering operation according to underlying vehicle running and steering conditions, thereby improving the control stability and maneuverability or the vehicle. One example of such improved steering systems is disclosed in Japanese Patent No. 2501606.
The disclosed steering system 300, as shown here in FIG. 11, includes a gear housing 303 connected via left and right links 302, 302 to a vehicle body 301 such that the gear housing 303 is displaceable in the widthwise direction of the vehicle body 301. The gear housing 303 is also supported by left and right elastic members 304, 304 on the vehicle body 301 such that the gear housing 303 is displaceable both in the radial direction and the widthwise direction thereof. The steering system 300 has an actuator 306 mounted to the vehicle body 301 via an elastic member 305. The actuator 308 has an actuating rod 307 connected to one end of an L-shaped arm 308, the other end of the arm being firmly connected to the gear housing 303.
The gear housing 303 contains within it a rack and pinion mechanism 311. The rack and pinion mechanism 311 has a pinion 315 connected to a steering wheel 312 via a steering shaft 313 and a set of shaft couplings 314, 314, and a rack 318 connected to steerable wheels (front wheels) 316, 316 via tie rods 317, 317.
With this arrangement, when the driver turns the steering wheel 312 in one direction, the steering system 300 operates to swivel the steerable wheels 316, 316 in the same direction via the rack and pinion mechanism 311. The same swivel motion of the steerable wheels 316, 316 can also occur when the actuator 306 operates to extend or contract its actuating rod 307 to move the gear housing 303 in a widthwise direction of the vehicle, thereby displacing the rack and pinion mechanism 311 as a whole in the same widthwise direction of the vehicle.
Here, the total amount of axial displacement (S30) of the rack 318 is represented by a combination or the amount of axial displacement (S31) of the rack 318 achieved by manual steering operation of the driver at the steering wheel 312 and the amount of axial displacement (S32) of the rack 318 achieved by operation of the actuator 306 (S30=S31xc2x1S32). This means that the actuator 306 can assist the manual steering operation. The actuator 306 can control the range of the steering ratio, which is represented by the ratio of the steering angle of the steering wheel 312 to the steered angle of the steerable wheels 316. The steering ratio is also called xe2x80x9csteering angle ratioxe2x80x9d.
The maximum controlled variable achieved by the actuator 306, that is, the amount of maximum axial displacement (332) of the rack 318 achieved by the actuator 306, is determined depending on the stroke of the actuating rod 307 and the maximum range of displacement of the gear housing 303. Thus, the amount of axial displacement (S32) of the rack 308 achieved by the actuator 306 is limited to a certain range.
The rack and pinion mechanism 311 of the conventional steering system 300 has a fixed gear ratio (the number of full turns of the pinion 315 required to move the rack 318 all the way from left to right).
Due to the fixed gear ratio of the rack and pinion mechanism 311, the rack gain of the conventional steering system 300 is always constant regardless or the steering angle, as shown in FIG. 12A. The rack gain is represented by the amount of axial displacement (mm) of the rack achieved when the pinion (that is, the steering wheel) makes a single complete turn. The rack gain is also called xe2x80x9crelative strokexe2x80x9d. In the table shown in FIG. 12A, the y-axis represents the rack gain (mm/turn) and the x-axis the steering angle (degrees) of the steering wheel 312. The midpoint on the x-axis represents the neutral position of the steering wheel, at which the steering angle of the steering wheel is 0 degree. In each rotational sense, the steering system 300 has the same range of steering angles.
FIG. 12B is a graph showing a steering ratio characteristic curve St1 of the conventional steering system 300. The y-axis of the graph is the Steering ratio (deg/deg) and the x-Axis of the graph is the steering angle (deg) of the steering wheel. The smaller the steering ratio, the larger the steered angle of the steerable wheels in relation to the steering angle of the steering wheel. The steering ratio characteristic represented by the curve St1 depends solely on the amount of axial displacement of the rack achieved by manual steering operation of the driver in the absence of the assistance by the actuator.
Because of the constant or fixed rack gain shown in FIG. 12A, the steering ratio characteristic curve St1 shown in FIG. 12B indicates that the steering ratio is maximum when the steering wheel is in the neutral position, and it becomes small as the steering angle increases. When the steering wheel is in the neutral position, the steering ratio is R2. When the steering wheel is in its left or right end position of A maximum steering angle xcex812, the steering ratio is R1, which is smaller than R2 (R1 less than R2).
Thus, a large (or high) steering ratio achieved in relation to a relatively small steering angle responds more slowly to the steering wheel, so that the steerable wheels are steered slowly. Conversely, a small (or low) steering ratio achieved in relation to a relatively large steering angle respond more quickly to the steering wheel, so that the steerable wheels are steered relatively quickly. This is due to a reason, which will be discussed below with reference to FIGS. 13A-13B and 14.
FIG. 13A is a diagrammatical plan view of a generally used vehicle steering system, and FIG. 13B is a diagrammatical side view of a steerable wheel of the steering system. In FIG. 13B, reference character Fr and Rr represent the forward direction and the backward direction, respectively, as viewed from the driver. As shown in FIG. 13A, the steering system 400 includes a rack and pinion mechanism 401 having a rack 402 connected at one end to one end or a tie rod 404 via a first universal joint 403, a knuckle arm 406 connected at one end to the other end of the tie rod 404 via a second universal joint 405, a kingpin 408 connected to the other end of the knuckle arm 404, and a steerable wheel 407 mounted to swivel or turn about the axis of the kingpin 408.
When a steering wheel 409 is manipulated or turned in one direction by the driver, manual steering force is transmitted successively through a pinion 401a and a rack gear 401 of the rack and pinion mechanism 401, the rack 402, the tie rod 404 and the knuckle arm 406 to the steerable wheel 407 so that the steerable wheel 407 is turned in the same direction as the steering wheel.
The steered angle of the steerable wheel 407 is a rotational angle about the axis of the kingpin 408 when viewed in the plan view. The three-dimensional length of the tie rod 404 is always constant. However, in FIG. 13A, the distance xcex11 from the axis of the kingpin 408 to the second universal joint 405 becomes small as the steered angle of the steerable wheel 407 approaches its maximum value. This is because when the steerable wheel 407 is viewed from an axial direction thereof, as shown in FIG. 13B, the axis of the kingpin 408 tilts toward the back with positive caster angle, and the knuckle arm 406 extends backward at right angles from the kingpin 409. The knuckle arm 406 turns about the kingpin 408 so that the length of the knuckle art 406 as measured in the plan view, that is, the distance xcex11 becomes small as the steerable wheel 407 approaches its lock position in each steering direction.
FIG. 14 is a plan view illustrative of the operation of the steering system shown in FIGS. 13A and 13B, the view showing the rack, tie rod and knuckle arm only. When the rack 402 is axially displaced in the arrowed direction, the first universal joint 403 located at the position P1 passes successively through the positions P1, P2, P3 and P4. These positions P1-P4 are equidistant from one another. The distance x1, x2, x3 between the adjacent positions P1, P2, P3 and P4 (corresponding to the amount of displacement of the rack 402 and the first universal joint 403) is proportional to the steering angle of the steering wheel 409 (FIG. 13A).
The second universal joint 405 and the knuckle arm 406 are angularly movable about the axis of the kingpin 408. When the rack 402 is axially displaced in the arrowed direction as previously described, the knuckle arm 406 located on the position Q1 moves consecutively from the positions Q1 to Q2, Q2 to Q3 and Q3 to Q4. For instance, when the first universal joint 403 moves from the position P1 to the position P2, the second universal joint 405 angularly moves from the position Q1 to the position Q2 through an angle of xcex11. Similarly, displacement of the first universal joint 403 from the position P2 to the position P3 causes the second universal joint 405 to angularly move from the position Q2 to the position Q3 through an angle of xcex12. Furthermore, displacement of the first universal joint 403 from the position P3 to the position P4 causes the second universal joint 405 to angularly move from the position Q3 to the position Q4 through an angle of xcex13.
The amount of displacement of the first universal joint 403 varies with uniform increments (x1=x2=x3), whereas the amount of angular displacement of the second universal joint 405 varies with non-uniform increments which become greater as the steerable wheel 407 approaches its steering lock position in either direction (xcex11 less than xcex12 less than xcex13). Thus, the steerable wheel 407 responds more quickly to the steering wheel as the steering angle of the steering wheel 409 (FIG. 13A) becomes large. By virtue of the steering geometry, the steering ratio characteristic curve St1 shown in FIG. 12B is obtained. In this connection, the length of the tie rod 46 as measured in the plan view also varied with the displacement of the rack 402; however, further description thereof is not necessary here.
Turning back to FIG. 12C, there is shown a steering ratio control characteristic curve which defines an optimum steering ratio control range A1 used for controlling the steering ratio by means of the actuator. In the table shown in FIG. 12C, the y-axis is the steering ratio (deg/deg) and the x-axis is the steering angle (deg) of the steering wheel. As indicated by hatching in FIG. 12C, the optimum steering radio control range A1 has a lower limit (control limit on the quick steer side) defined by the steering ratio characteristic curve St1 shown in FIG. 12B, and an upper limit (control limit on the slow steer side) defined by a controllable upper limit steering ratio characteristic curve St2. The optimum steering radio control range A1 has a width B1. The controllable upper limit steering ratio characteristic curve St2 is drawn on the basis of the total amount of displacement of the rack achieved with the assistance of control operation of the actuator. This curve St2 is offset upward from the steering ratio characteristic curve St1 (FIG. 12B) by a distance of the control ratio width B1.
The steering ratio control range A1 represents a range in which the steering ratio can be controlled according to the total amount of displacement (S30) of the rack which is obtained by subtracting the amount of displacement (S32) of the rack achieved by operation of the actuator, from the amount of displacement (S31) of the rack achieved by manual steering operation of the driver effected on the steering wheel (S30=S31xe2x88x92S32). Thus, with the steering ratio characteristic curve St1 used as a control reference on the quick steer side, the actuator can control the slow steer of the steerable wheel. Since S30=S31xe2x88x92S32 as discussed previously, this means that the amount of driver-dependent displacement S31 of the rack can be increased by subtracting an inverse of the amount of actuator-dependent displacement S32 or the rack from S31 (that is, by moving the rack by the actuator in a direction opposite to the direction intended by the steering wheel).
FIG. 15A is a graphical representation or the steered angle control characteristic of the conventional steering device. In this figure, the y-axis of the graph is the steered angle controlled variable (deg), and the x-axis of the graph is the steering angle (deg) of the steering wheel. The midpoint on the x-axis represents the neutral position of the steering wheel, at which the steering angle of the steering wheel is 0 degree. The steered angle controlled variable represents the controlled variable indicated in terms of the steered angle, which is used when controlling the steered angle of the steerable wheel by displacing the rack by the actuator.
As mentioned previously, the rack gain is always constant due to the fixed gear ratio of the rack and pinion mechanism, and the controlled variable that can be achieved by reducing amount of actuator-dependent axial displacement S32 of the rack is limited to a certain range. It is evident from FIG. 15A that when the steering angle of the steering wheel is xcex811, the steered angle controlled variable becomes maximum with a value C1, and this maximum control variable C1 continues even with an increase in the steering angle until a steering angle xcex812 is reached. This means that the steering angles in the range of xcex811 to xcex811 extending across the neutral position can only be effective to control the steered angle of the steerable wheels.
FIG. 15B is a graph showing a steering ratio control characteristic curve of the conventional steering system. The y-axis of the graph is the steering ratio (deg/deg) and the x-axis of the graph is the steering angle (deg) of the steering wheel. As shown in this figure, the steering ratio control characteristic curve defies a practical steering ratio control range A2, which is achieved when the range of steering ratio is controlled by the actuator. The practical steering ratio control range A2, like the optimum steering ratio control range A1 shown in FIG. 12C, is defined by the steering ratio characteristic curve St1 and the controllable upper limit steering ratio characteristic curve St2.
The steered angle control characteristic curve shown in FIG. 15A is used in combination with the steering ratio characteristic curve St1 so thereby draw or prepare the controllable upper limit steering ratio characteristic curve St2. Thus, the gradient of the controllable upper limit steering ratio characteristic curve St2 agrees with that of the steered angle control characteristic curve.
However, since the steered angle controlled variable is avail able only for the steering angles in the range of xe2x88x92C1 to +C1, no response can be obtained for steering angles in the range of xcex811 to xcex812. Due to the absence of the steered angle controlled variable, the practical steering ratio control range A2 is narrowed at a region corresponding to the relatively large steering angles xcex811 to xcex812. With this narrowing, an uncontrollable or inert area A3 is formed as indicated by broken lines in FIG. 15B. Due to the presence of the inert area A3, the practical steering ratio control range A2, as opposed to the optimum steering ratio control range A1 shown in FIG. 12C, cannot respond to the steering wheel over the fall range of steering angles.
Despite the limited steered angle controlled variable, a response to the full range of steering angles may be possible by narrowing the control ratio width B1 between the curves St1 and St2. However, this measure is not practical because the resulting controllable range of the steering ratios is very small.
It is accordingly an object of the present invention to provide a vehicle steering system, which is capable of controlling the steering ratio relative to a wider range or steering angles even when only a limited controlled variable is available.
To achieve the foregoing object, according to the present invention, there is provided a vehicle steering system comprising a steering wheels a rack and pinion mechanism having a pinion functionally coupled to the steering wheel and a rack meshing with the pinion and functionally coupled to steerable wheels; a housing accommodating within it the rack and pinion mechanism; and
means for displacing the rack in the axial direction thereof to steer the steerable wheels independently of steering operation initiated by the steering wheel. The rack and pinion mechanism comprises a variable gear ratio rack and pinion mechanism having a variable gear ratio, which is the lowest when the steering wheel is in a neutral position and becomes higher as the steering angle of the steering wheel becomes large.
It is preferable that the rack has a first region generally corresponding in position to the neutral position of the steering wheel, and a second region extending contiguously from the first region in one direction along the axis of the rack, the first region has a fixed tooth pitch, and the second region has a variable tooth pitch smaller than the fixed pitch of the first region and reducing progressively in a direction from the first region toward an end of the second region opposite from the first region.
The rack may further have a third region extending from the end of the second region along the axis of the rack and generally corresponding in position to an end portion of an available range of the steering angles located remote from the neutral position, the third region having a fixed tooth pitch smaller than that of the second region.
In one preferred form of the present invention, the means for displacing the rack comprises: support means for supporting the housing on a vehicle body such that the housing is displaceable relative to the vehicle body in the widthwise direction of the vehicle body; and an actuator functionally coupled to the housing and operating to displace the housing relative to the vehicle body in the widthwise direction of the vehicle body, thereby causing the rack to move together with the housing in the width wise direction of the vehicle body. It is preferable that the support means comprises a link mechanism interconnecting the housing and the vehicle body such that the link mechanism together with the housing and a part of the vehicle body forms a quadric parallel linkage, and the actuator has an output portion operatively connected to the link mechanism.
The means for displacing the rack may further comprise a power transmitting mechanism disposed between the link mechanism and the actuator for transmitting power from the actuator to the link mechanism, the power transmission mechanism comprising a hypoid gear mechanism having a small gear connected to the output portion of the actuator and a large gear meshing with the small gear, and a drive link having one end pivotally connected to the link mechanism and the opposite end pivotally connected to the large gear in eccentric relation to the large gear.
In another preferred form of the prevent invention, the means for displacing the rack comprises: support means for supporting the pinion within the housing such that the pinion is displaceable in the axial direction of the rack; and an actuator functionally coupled to the pinion and operating to displace the pinion in the axial direction of the rack, thereby causing the rack to move together with the pinion in the axial direction thereof. It is preferable that the support means comprises a swing arm pivotally connected at one end to the housing and rotatably supporting thereon the pinion the swing arm extending transversely across the rack, and the actuator is mounted to the housing and has an output portion operatively connected to a free end of the swing arm.
The means for displacing the rack may further comprise a power transmitting mechanism disposed between the swing arm and the actuator for transmitting power from the actuator to the sting arm, the power transmission mechanism comprising a hypoid gear mechanism having a small gear connected to the output portion of the actuator and a large gear meshing with the small gear, and a drive link having on end pivotally connected to the free end of the swing arm and the opposite end pivotally connected to the large gear in eccentric relation to the large gear.
The swing arm may have a hollow structure having an internal space formed therein, the rack extending through the hollow space of the swing arm.
According to the present invention, the steerable wheels can be steered also by displacing the housing or the pinion relative to the vehicle body in the axial direction of the rack. With this arrangement, the total amount of axial displacement of the rack is represented by a combination of the amount of axial displacement of the rack achieved by manual steering operation effected at the steering wheel by the driver, and the amount of displacement of the rack achieved with the displacement of the housing or the pinion. Thus, by controlling the steered angle of the steerable wheels according to the displacement of the housing or the pinion, control of the steering ratio is possible. However, due to a limited displacement of the housing, a controllable range of the steering ratio in restricted too.
To deal with this problem, the rack and pinion mechanism of the present invention comprises a variable gear ratio type rack and pinion mechanism having a variable gear ratio, which is the lowest when the steering wheel is in a neutral position and becomes higher au the steering angle of the steering wheel becomes large. With the variable gear ratio type rack and pinion mechanism, the amount of axial displacement of the rack caused by one complete turn of the steering wheel is made to decrease inversely with the steering angle. Thus, the amount of displacement of the housing or pinion per single revolution of the steering wheel decreases, correspondingly. Accordingly, in spite of a limited amount of displacement of the housing or pinion being available, the steering ratio can be controlled extensively with respect to the steering angle.